Tight footprint white light emitting diode (LED) packages usually include at least one conformal coating of phosphor applied on the LED chips. However, the vertical emission profile of the light source is limited by the height of the chip substrate. Also due to the side emission of the substrate, the phosphor is advantageously coated on the vertical sidewall to capture all the light. Phosphor coating on the sidewall can result in poor goniometric color uniformity due to phosphor thickness non-uniformity. Another disadvantage of a conformally coated small light source is heat-induced degradation of the host material for the phosphor layer, due to the fact that the phosphor blend is close to the chip and absorbs more radiometric power per unit area and in turn generate more heat per unit area. However, heat dissipation of the phosphor layer is poor due to lack of heat conduction.
To address these problems, it is known to use a light emitting package that employs a plurality of light emitting diode chips. Using several chips provides higher light output, and if the colors of the chips are suitably selected (for example, by selecting red, green, and blue chips or a similar combination of saturated colors) the light output can approximate white light. Alternatively, it is known to employ light emitting diode chips that emit in the blue, violet, or ultraviolet range, and to coat such chips with a suitable phosphor blend to approximate white light. The output can be substantially completely converted light (e.g., ˜100% conversion efficiency with the phosphor producing approximately white light) or can be a blend of direct chip emission and converted phosphor emission (e.g., blue direct chip emission and yellowish converted phosphor emission that blend to approximate white light).
In addition, yield of wafer level substrate lift off is very low due to epitaxial layer cracking after lift-off. The yield is expected to be worse when larger size substrates are used.